Monitoring the levels of toxic metals of atmospheric particulate matter in the Rustenburg district

Abstract

Ambient air quality has been monitored in South Africa for the last four decades using a variety of methods. There is however, no agreed method and system in function to monitor the levels of heavy metals in ambient air in South Africa. The objective of this study was to determine the concentration levels of toxic metals (Cr, Ni, V, and Pb) of air particulate matter in the mining areas of the North-West Province, and to suggest an air particulate monitoring method and sampling system. A comprehensive literature study was conducted on aerosol dynamics, the effects of PM on humans and the environment, and the existing monitoring methods used worldwide. The literature review showed that large variations in concentration levels can be observed under different meteorological conditions, and that meaningful source apportionment can be achieved by performing chemical analysis of the trace element component of particulate matter. The South African situation with regard to monitoring of particulate matter herein studied, suggested the need for the development of characterisation methods for particulate matter. The concentration levels of air particulate matter PM10 were determined using Tapered Element Oscillating Microbalance (TEOM). The hourly concentrations of PMIO were measured in the range from 13.12 to 215.7 µg.m-3, the daily levels from 10.3 to 151.7 µg.m-3 and the monthly levels from 22.2 to 131.0 µg.m-3. The concentrations of the oxides of trace metals were found from analysis by Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS), to be in the range of 0.03 - 3.8 µg.m-3 for Cr, 0.01 - 0.03 µg.m-3 for Ni, 0.00 - 0.02 µg.m-3 for V and 0.00 - 0.24 µg.m-3 for Pb. The monthly concentrations of the toxic trace metals of particulate matter measured using Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), were in the range of 0.03 - 5.2 µg.m-3 for Cr, 0.03 - 2.8 µg.m-3 for Ni, 0.01 - 0.05 µg.m-3 for V and 0.02 - 0.5 µg.m-3 for Pb. ICP-MS was shown to be a relevant characterisation tool for particulate aerosols. Sources of metals of particulate matter within the Rustenburg area were successfully apportioned (using correlation and regression analysis, and principal component analysis) in order of decreasing abundance as soil dust, mining industry, traffic and biomass burning, unknown sources, other industries and smelting. A comparative assessment of the efficiency and relevance of the sampling and analysis methods used in this study was performed. The efficiency of the type of filter media used, the frequency of sampling, the sampling period, the meteorology and the locations of the sampling sites used were assessed. Recommendations were made that the monitoring procedure and system of air particulate matter in the North-West province and South Africa at large, should not be based only on determining the hourly, daily and annual levels of particulate matter for compliance with air quality standards but should also include determination of the chemical and physical properties of atmospheric pollutants, estimation of the emitting sources, and evaluation of health effects on the exposed community as well as assessment of the environmental impacts. The approved sampling and analysis methods for the province should be comprehensive for both the polluter and the air pollution authorities, and specific and relevant to the type of pollutants within the province.